Bunny, Bunny,” a charming play directed by Warner Crocker about the late comedienne Gilda Radner’s platonic relationship with writer Alan Zweibel, currently at the Mercury Theater, 3745 N. Southport Ave., has been extended to April 15.It has been billed as “sort of a love story,” but it is much more than that.It is a lovingly told tale of a bittersweet romance, narrated in a most convincing portrayal of Zweibel (who wrote the play) by Jackson Peter Evans as someone who absolutely adored her from …